1. Field of the Invention
The present invention relates to a power supply control apparatus including an overcurrent detection circuit.
2. Description of Related Art
In recent years, what is called an IPD (Intelligent Power Device) is used in electronic control systems for vehicle. The IPD includes, as a switch element for driving a load such as a lamp and motor, a control circuit and a switching element such as power MOSFET (Metal-Oxide Semiconductor Field-Effect Transistor) or IGBT (Insulated Gate Bipolar Transistor). In these systems including loads and the IPD, for example, when there occurs a trouble such as terminal short circuit in a terminal of the electronic control system, wiring short circuit or load short circuit, overcurrent may flow through the wire harness and the switching element (power MOSFET or the like) of the IPD, thus causing damage. Thus, as a control circuit for the IPD, a circuit (overcurrent protection circuit) is typically provided which detects overcurrent and turns off the power MOSFET. Here, in order to adequately protect the load and the power MOSFET, there is a need for a high precision power supply control apparatus.
Recently, as a technique for the overcurrent protection circuit, for example, a power supply control apparatus has been proposed which is described in Japanese Patent Laid-Open No. 2005-39573 (Reference 1) and its counterpart, U.S. Patent Application Publication No. 2005/0013079 A1 (Reference 2). FIG. 7 illustrates a power supply control apparatus 700 described in References 1 and 2. The power supply control apparatus 700 includes an output MOS transistor MQ1 used to switch on/off power supplied from a power supply line 101 to a load 102. A drain terminal of the output MOS transistor (power MOSFET) MQ1 is connected to a power source terminal 103 leading to the power supply line 101. A source terminal of the output MOS transistor MQ1 is connected to an output terminal 104 leading to the load 102. A gate terminal of the output MOS transistor MQ1 is connected to a control circuit 105 which outputs a control signal for switching on/off the output MOS transistor MQ1 (or supplies a control voltage to the same). The load 102 is connected to a ground line 106 (for example, a vehicle frame).
The power supply control apparatus 700 further includes a current detection MOS transistor MQ2 which has a structure similar to the output MOS transistor MQ1 (that is, being different only in dimensions and equal in characteristic per unit channel width). The respective drain terminals of the current detection MOS transistor MQ2 and the output MOS transistor MQ1 are connected in common to the power source terminal 103; and the respective gate terminals are connected in common to the control circuit 105. The power supply control apparatus 700 further includes a current detection resistor MRS connected in series between the source terminals of the current detection MOS transistor MQ2 and the output MOS transistor MQ1.
The power supply control apparatus 700 further includes MOS transistors MQ3 and MQ4 which constitute a current mirror. A source terminal of the MOS transistor MQ3 is connected to a connecting node 107 between the current detection resistor MRS and the current detection MOS transistor MQ2. A gate terminal and a drain terminal of the MOS transistor MQ3 are connected in common to a connecting node 111 and also connected to a drain terminal of a MOS transistor 109. A source terminal of the MOS transistor MQ4 is connected to a connecting node 108 between the source terminal of the output MOS transistor MQ1 and the current detection resistor MRS. Further, a gate terminal of the MOS transistor MQ4 is connected in common to the connecting node 111. A drain terminal of the MOS transistor MQ4 is connected via a connecting node 112 to a drain terminal of a MOS transistor 110. The respective gate terminals of the MOS transistors 109 and 110 are connected in common to a bias signal supply source; and the respective source terminals thereof are connected in common to a power source terminal 103. An overcurrent detection signal is extracted from the connecting node 112.
Here, consider a case in which, due to some reason, a line for connecting the load 102 gets loose and causes a short circuit with the vehicle frame, or the terminal 104 in the electronic control system comes into contact with the ground line. In this case, a short circuit is formed via the output MOS transistor MQ1 between the power supply line 101 and the ground line 106, and overcurrent flows through the output MOS transistor MQ1, which is an abnormal state. When such abnormal state occurs, there is a need to turn off the output MOS transistor MQ1, or to suppress the current flowing through the output MOS transistor MQ1, so that the output MOS transistor MQ1 is protected. This overcurrent detection operation will be briefly described below.
The output MOS transistor MQ1 controls switching on/off of a power supply voltage supplied from the power supply line 101 to the load 102. That is, a control signal output from the control circuit 105 controls the connection between the drain terminal and source terminal of the output MOS transistor MQ1. The output MOS transistor MQ1 has a structure similar to the current detection MOS transistor MQ2, so when the current flowing through the output MOS transistor MQ1 increases (10 A, for example), the current flowing through the current detection MOS transistor MQ2 also increases according to the homothetic ratio (10000:1, for example) between the output MOS transistor MQ1 and the current detection MOS transistor MQ2 (for example, 10 A/10000=1 mA). Accordingly, potential Vs at the connecting node 107 and potential V1 at the connecting node 111 rise. Consequently, the current flowing between the drain terminal and source terminal of the MOS transistor MQ4 increases. Here, the MOS transistor MQ3 and the MOS transistor MQ4 have a similar structure.
When the current flowing between the drain terminal and source terminal of the MOS transistor MQ4 exceeds a reference current value Iref2 (50 for example) set by the MOS transistor 110, the overcurrent detection signal output via the connecting node 112 changes from a high level to a low level, so it is determined that the output MOS transistor MQ1 is in an overcurrent state. However, when the current flowing through the output MOS transistor MQ1 is small, the on-current flowing through the MOS transistor MQ4 is smaller than the reference current value Iref2. In this case, the overcurrent detection signal output via the connecting node 112 keeps the high level, so it is determined that the output MOS transistor MQ1 is not in an overcurrent state.
Here, the inventor has found that the power supply control apparatus 700 can be still improved. In the power supply control apparatus 700, overcurrent detection is performed based on a reference current (for example, a current Iref2 flowing through the MOS transistor 110) used as a reference for determining occurrence of overcurrent state and on a current (for example, current flowing through the current detection MOS transistor MQ2) flowing through the output MOS transistor MQ1. Thus, there is a need to adjust the size of the transistors (for example, MOS transistors MQ3 and MQ4, and MOS transistors 109 and 110). That is, the transistors have a structure similar to each other, but are different in size. Accordingly, the transistors are affected by characteristic variations in manufacturing process or by characteristic variations dependent on ambient temperature conditions. Further, effects of bias signal accuracy should be also taken into consideration.